1. Technical Field
The present invention is directed to a system for enhancing a speech signal in a noisy environment through corrective adjustment of spectral noise power density estimations.
2. Related Art
Speech signals obtained through a microphone may include ambient noise. This noise may be added to the desired speech signal and may result in a corresponding distorted signal that includes both the desired speech signal and ambient noise signal. In hands free telephony, the distorted signal may include the voice signal, background noise, and echo components. In the case of a vehicle, the background noise may include the noise of the engine, the windstream, and the rolling tires. Unwanted signal components, such as echoes, may also be present in the distorted signal due to sound from loudspeakers connected to a radio and/or a hands-free telephony system.
A speech signal that includes noise may impair the use of the speech signal in some applications. The performance of speech recognition software may be diminished where the speech signal also includes noise. In hands free telephony applications, noise may reduce communication quality and intelligibility.
Noise reduction filters may be used to extract the desired speech signal from unwanted noise. The distorted signal may be split into frequency bands by a filter bank in the frequency domain. Noise reduction may then be performed in each frequency band separately. The filtered signal may be synthesized from the modified spectrum by a synthesizing filter bank, which transforms the signal back into the time domain.
Noise reduction filters may use estimates of the spectral power density of the distorted signal and of the noise component to extract the desired speech signal from the unwanted noise. Depending on the ratio of both quantities, a weighting factor may be applied in the distorted frequency band. The relationship between the spectral signal power and the weighting factor may be influenced by the filter characteristics. Filter performance may rely on an accurate estimate of the spectral noise power density. Inaccurate estimations of the spectral power density of the noise component may result in unwanted artifacts, including artifacts that may occur during interruptions in the speech signal.